U.S. patent application number 10/867755 was filed with the patent office on 2005-07-21 for flip chip nitride semiconductor light emitting diode.
Invention is credited to Jeong, Young June, Kim, Hyun Kyung, Kim, Yong Chun.
Application Number | 20050156185 10/867755 |
Document ID | / |
Family ID | 34747876 |
Filed Date | 2005-07-21 |
United States Patent
Application |
20050156185 |
Kind Code |
A1 |
Kim, Hyun Kyung ; et
al. |
July 21, 2005 |
Flip chip nitride semiconductor light emitting diode
Abstract
The invention relates to a nitride semiconductor LED, wherein an
n-doped nitride semiconductor layer is formed on a transparent
substrate. An active layer is formed on the n-doped nitride
semiconductor layer. A p-doped nitride semiconductor layer is
formed on the active layer. A high reflectivity Ohmic contact layer
of a mesh structure is formed on the p-doped nitride semiconductor
layer and has a number of open areas for exposing the p-doped
nitride semiconductor layer. A metal barrier layer is formed on at
least a top region of the high reflectivity Ohmic contact layer. A
p-bonding electrode is formed on the metal barrier layer. An
n-electrode is formed on the n-doped nitride semiconductor
layer.
Inventors: |
Kim, Hyun Kyung;
(Kyungki-do, KR) ; Jeong, Young June; (Kyungki-do,
KR) ; Kim, Yong Chun; (Kyungki-do, KR) |
Correspondence
Address: |
LOWE HAUPTMAN GILMAN AND BERNER, LLP
1700 DIAGONAL ROAD
SUITE 300 /310
ALEXANDRIA
VA
22314
US
|
Family ID: |
34747876 |
Appl. No.: |
10/867755 |
Filed: |
June 16, 2004 |
Current U.S.
Class: |
257/99 ;
257/E33.068 |
Current CPC
Class: |
H01L 33/44 20130101;
H01L 33/32 20130101; Y10S 257/918 20130101; H01L 2224/14 20130101;
H01L 33/387 20130101; H01L 33/405 20130101 |
Class at
Publication: |
257/099 |
International
Class: |
H01L 033/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 19, 2004 |
KR |
2004-3960 |
Claims
1. A nitride semiconductor light emitting diode comprising: a
transparent substrate for the single crystal growth of nitride; an
n-doped nitride semiconductor layer formed on the transparent
substrate; an active layer formed on the n-doped nitride
semiconductor layer; a p-doped nitride semiconductor layer formed
on the active layer; a high reflectivity Ohmic contact layer of a
mesh structure formed on the p-doped nitride semiconductor layer
and having a number of open areas for exposing the p-doped nitride
semiconductor layer; a metal barrier layer formed on at least a top
region of the high reflectivity Ohmic contact layer; a p-bonding
electrode formed on the metal barrier layer; and an n-electrode
formed on the n-doped nitride semiconductor layer.
2. The nitride semiconductor light emitting diode according to
claim 1, wherein the open areas of the high reflectivity Ohmic
contact layer total up to 50% or less of the entire top area of the
Ohmic contact layer.
3. The nitride semiconductor light emitting diode according to
claim 1, wherein the high reflectivity Ohmic contact layer has a
reflectivity of at least 70%.
4. The nitride semiconductor light emitting diode according to
claim 1, wherein the high reflectivity Ohmic contact layer
comprises at least one layer made of a material selected from a
group including Ag, Ni, Al, Ph, Pd, Ir, Ru, Mg, Zn, Pt, Au and
combinations thereof.
5. The nitride semiconductor light emitting diode according to
claim 1, wherein the high reflectivity Ohmic contact layer
comprises a first layer made of a material selected from a group
including Ni, Pd, Ir, Pt and Zn and a second layer of Ag or Al
formed on the first layer.
6. The nitride semiconductor light emitting diode according to
claim 1, wherein the high reflectivity Ohmic contact layer
comprises a first layer made of Ni, a second layer of Ag formed on
the first layer and a third layer of Pt formed on the second
layer.
7. The nitride semiconductor light emitting diode according to
claim 5, wherein the first layer has a thickness ranging from about
5 to 50 .ANG., the second layer has a thickness ranging from about
1000 to 10000 .ANG., and the third layer has a thickness ranging
from about 100 to 500 A.
8. The nitride semiconductor light emitting diode according to
claim 1, wherein the metal barrier layer is formed to surround the
entire high Ohmic contact layer.
9. The nitride semiconductor light emitting diode according to
claim 1, wherein the metal barrier layer is formed to be connected
with top regions of the p-doped nitride semiconductor layer exposed
by the open areas of the high reflectivity Ohmic contact layer.
10. The nitride semiconductor light emitting diode according to
claim 1, wherein the metal barrier layer comprises at least one
layer made of a material selected from a group including Ni, Al,
Cu, Cr, Ti and combinations thereof.
11. The nitride semiconductor light emitting diode according to
claim 1, further comprising a dielectric barrier layer formed on
the high reflectivity Ohmic contact layer to expose at least the
p-bonding electrode.
12. The nitride semiconductor light emitting diode according to
claim 11, wherein the dielectric barrier layer is formed to
surround the high reflectivity Ohmic contact layer.
13. The nitride semiconductor light emitting diode according to
claim 11, wherein the dielectric barrier layer is formed on one
side of the light emitting diode to expose the p-bonding electrode
and the n-electrode.
14. The nitride semiconductor light emitting diode according to
claim 11, wherein the dielectric barrier layer comprises a
reflection layer having two types of dielectric layers of different
refractivity, the two types of dielectric layers repeatedly
alternating with each other.
15. The nitride semiconductor light emitting diode according to
claim 11, wherein the dielectric barrier layer is made of oxide or
nitride that contains an element selected from a group including
Si, Zr, Ta, Ti, In, Sn, Mg and Al.
16. A flip chip nitride semiconductor light emitting diode, which
has n- and p-doped nitride semiconductor layers formed on a
substrate, comprising: a high reflectivity Ohmic contact layer of a
mesh structure formed on the p-doped nitride semiconductor layer
and having a plurality of open areas for exposing the p-doped
nitride semiconductor layer; a metal barrier layer formed on the
high reflectivity Ohmic contact layer; and a p-bonding electrode
formed on the metal barrier layer.
Description
CLAIM OF PRIORITY
[0001] This application claims the benefit of Korean Patent
Application No. 2004-3960 filed on Jan. 19, 2004, in the Korean
Intellectual Property Office, the disclosure of which is
incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a nitride semiconductor
Light Emitting Diode (LED), and more particularly, to a flip chip
nitride semiconductor LED of excellent electric characteristics and
luminance.
[0004] 2. Description of the Related Art
[0005] Recently, a nitride semiconductor LED as an optical device
for generating blue or green wavelength light is made from
semiconductor material expressed by an equation of
Al.sub.xIn.sub.yGa.sub.(1-x-y)N (wherein 0.ltoreq.x.ltoreq.1,
0.ltoreq.y.ltoreq.1 and 0.ltoreq.x+y.ltoreq.1). Considering lattice
match, nitride semiconductor crystals are grown on a substrate such
as a sapphire substrate that is used for the growth of nitride
single crystals. Since the sapphire substrate is electrically
insulated, p- and n-electrodes are formed on the same side of a
final nitride semiconductor LED.
[0006] According to structural characteristics as above, nitride
semiconductor LEDs have been positively developed into specific
geometries adequate to flip chip structures. FIG. 1 illustrates a
flip chip light emitting device having a conventional nitride
semiconductor LED mounted thereon.
[0007] A flip chip light emitting device 20 shown in FIG. 1
includes a nitride semiconductor LED 10 mounted on a supporting
substrate 21. The nitride semiconductor LED 10 includes a sapphire
substrate 11 and an n-doped nitride semiconductor layer 12, an
active layer 13 and a p-doped nitride semiconductor layer 14 formed
in their order on the sapphire substrate 11. The nitride
semiconductor LED 10 may be mounted on the supporting substrate 21
by welding electrodes 19a and 19b with lead patterns 22a and 22b
via conductive bumps 24a and 24b, respectively. In the flip chip
light emitting device 20 of this structure, the sapphire substrate
11 of the LED 10 may be used as a light emitting plane since it is
transparent.
[0008] Each electrode, in particular, the p-electrode of the flip
chip nitride semiconductor LED is required to have high
reflectivity for reflecting emission light from the active layer 13
toward the light emitting plane while forming an Ohmic contact with
the p-doped nitride semiconductor layer 14 as shown in FIG. 1.
Therefore, as shown in FIG. 1, a p-electrode structure may include
a high reflectivity Ohmic contact layer 15 formed on the p-doped
nitride semiconductor layer 14 and a metal barrier layer 16 for
preventing the diffusion of components of the Ohmic contact layer
15.
[0009] However, since the nitride semiconductor LED 10 shown in
FIG. 1 has a planar electrode structure, and in particular, the
p-electrode side Ohmic contact layer 15 has a lower specific
resistance (for example 5 to 10 m.OMEGA./cm.sup.2) lower than that
of the p-doped nitride semiconductor layer 14, the nitride
semiconductor LED 10 of this type has current crowding in which a
major portion of current flowing along the Ohmic contact layer 15
is concentrated in a narrow part A adjacent to the n-electrode as
indicated with an arrow.
[0010] Such current crowding increases forward voltage while
lowering the luminous efficiency of an active layer portion
relatively remote from the n-electrode to degrade luminance
properties. Further, the current concentrated part A generates a
large quantity of heat thereby remarkably degrading the reliability
of the LED.
SUMMARY OF THE INVENTION
[0011] The present invention has been made to solve the foregoing
problems of the prior art and it is therefore an object of the
present invention to provide a nitride semiconductor LED having an
improved p-electrode structure capable of reducing current crowding
in order to realize lower forward voltage as well as higher
luminous efficiency.
[0012] According to an aspect of the invention for realizing the
object, there is provided a nitride semiconductor light emitting
diode comprising: a transparent substrate for the single crystal
growth of nitride; an n-doped nitride semiconductor layer formed on
the transparent substrate; an active layer formed on the n-doped
nitride semiconductor layer; a p-doped nitride semiconductor layer
formed on the active layer; a high reflectivity Ohmic contact layer
of a mesh structure formed on the p-doped nitride semiconductor
layer and having a number of open areas for exposing the p-doped
nitride semiconductor layer; a metal barrier layer formed on at
least a top region of the high reflectivity Ohmic contact layer; a
p-bonding electrode formed on the metal barrier layer; and an
n-electrode formed on the n-doped nitride semiconductor layer.
[0013] The open areas of the high reflectivity Ohmic contact layer
may total up to preferably 70% or less, and more preferably, 50% or
less of the entire top area of the Ohmic contact layer. Further,
the high reflectivity Ohmic contact layer preferably has a
reflectivity of at least 70%.
[0014] Preferably, the high reflectivity Ohmic contact layer may be
made of a material selected from a group including Ag, Ni, Al, Ph,
Pd, Ir, Ru, Mg, Zn, Pt, Au and combinations thereof.
[0015] In more detail, the high reflectivity Ohmic contact layer
may comprise a first layer made of a material selected from a group
including Ni, Pd, Ir, Pt and Zn and a second layer of Ag or Al
formed on the first layer. Alternatively, the high reflectivity
Ohmic contact layer may comprise a first layer made of Ni, a second
layer of Ag formed on the first layer and a third layer of Pt
formed on the second layer.
[0016] In an embodiment of the high reflectivity Ohmic contact
layer of two or three layered structure, it is preferred that the
first layer has a thickness ranging from about 5 to 50 .ANG., the
second layer has a thickness ranging from about 1000 to 10000
.ANG., and the third layer has a thickness ranging from about 100
to 500 .ANG..
[0017] According to another embodiment of the invention, the metal
barrier layer may be formed to surround the entire high Ohmic
contact layer. Also, the metal barrier layer may be formed to be
connected with top regions of the p-doped nitride semiconductor
layer exposed by the open areas of the high reflectivity Ohmic
contact layer in order to function as another reflection layer in
the open areas of the high reflectivity Ohmic contact layer.
[0018] According to further another embodiment of the invention,
the nitride semiconductor light emitting diode may further comprise
a dielectric barrier layer formed on the high reflectivity Ohmic
contact layer to expose at least the p-bonding electrode.
[0019] The dielectric barrier layer may be formed to surround the
high reflectivity Ohmic contact layer, and on one side of the light
emitting diode to expose the p-bonding electrode and the
n-electrode as a conventional passivation layer. In particular, the
dielectric barrier layer of the invention may comprise a reflection
layer having two types of dielectric layers of different
refractivity, the two types of dielectric layers repeatedly
alternating with each other, in order to function as a high
reflectivity dielectric mirror layer. Preferably, the dielectric
barrier layer may be made of oxide or nitride that contains an
element selected from a group including Si, Zr, Ta, Ti, In, Sn, Mg
and Al.
[0020] The terminology used herein "flip chip nitride semiconductor
Light Emitting Diode (LED)" indicates an LED used in a flip chip
light emitting device in which a side of the LED having p- and
n-electrodes is mounted on a substrate of the light emitting
device.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] The above and other objects, features and other advantages
of the present invention will be more clearly understood from the
following detailed description taken in conjunction with the
accompanying drawings, in which:
[0022] FIG. 1 is a side sectional view illustrating a light
emitting device having a conventional nitride semiconductor LED
mounted thereon;
[0023] FIG. 2A is a side sectional view illustrating a flip chip
nitride semiconductor LED according to a first embodiment of the
invention;
[0024] FIG. 2B is a plan view of FIG. 2A;
[0025] FIG. 3A is a side sectional view illustrating a flip chip
nitride semiconductor LED according to a second embodiment of the
invention;
[0026] FIG. 3B is a side sectional view illustrating a chip
structure having the flip chip nitride semiconductor LED in FIG. 3A
mounted thereon;
[0027] FIG. 4A is a side sectional view illustrating a flip chip
nitride semiconductor LED according to a third embodiment of the
invention;
[0028] FIG. 4B is an enlargement of a part B of a dielectric
barrier layer shown in FIG. 4A; and
[0029] FIGS. 5A and 5B are graphs comparing forward and output
voltage characteristics of flip chip light emitting devices
incorporating nitride semiconductor LEDs of the invention,
respectively, with those of a conventional flip chip light emitting
device.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0030] Hereinafter components of a nitride semiconductor LED of the
invention will be described in more detail.
[0031] P- and N-Doped Nitride Semiconductor Layers
[0032] P- and n-doped nitride semiconductor layers which are single
crystals expressed by an equation of
Al.sub.xIn.sub.yGa.sub.(1-x-y)N (wherein 0.ltoreq.x.ltoreq.1,
0.ltoreq.y.ltoreq.1 and 0.ltoreq.x+y.ltoreq.1) can be grown via the
Metal Organic Chemical Vapor Deposition (MOCVD), the Molecular Beam
Epitaxy (MBE), the Hydride Vapor Phase Epitaxy (HVPE) and so on.
Representative examples of the nitride semiconductor layer may
include GaN, AlGaN or GaInN.
[0033] A p-doped semiconductor layer may contain dopants such as
Mg, Zn and Be, and an n-doped semiconductor layer may contain
dopants such as Si, Ge, Se, Te and C. In general, a buffer layer
may be formed between the n-doped nitride semiconductor layer and a
substrate. Typical examples of the buffer layer may include a low
temperature nucleation layer of for example AlN or GaN.
[0034] Active Layer
[0035] The active layer of the invention is adopted to emit
blue-green light (ranging from about 350 to 550 nm wavelength), and
made of an undoped nitride semiconductor layer having a single or
multiple quantum well structure. Similar to the p- or n-doped
nitride semiconductor layer, the active layer may be grown via any
of the MOCVD, the MBE, the HVPE and so on.
[0036] High Reflectivity Ohmic Contact Layer
[0037] As described hereinbefore, the high reflectivity Ohmic
contact layer of the invention is constituted of a mesh structure
having a number of open areas. The mesh structure of the high
reflectivity Ohmic contact layer relatively lengthens the current
path from a p-bonding electrode along the Ohmic contact layer
toward an n-electrode. As a result, this alleviates current
crowding in a specific region of an LED adjacent to the n-electrode
when the LED is energized. This also increases the ability of
current to flow toward the p-dope nitride semiconductor layer
thereby to relieve the problem of current crowding.
[0038] Further, the high reflectivity Ohmic contact layer is
necessarily made of a suitable material to lower the contact
resistance between itself and the p-doped nitride semiconductor
layer of a relatively high energy band gap, and preferably, a high
reflectivity material in consideration of a structural aspect of
the flip chip nitride semiconductor LED.
[0039] The high reflectivity Ohmic contact layer may be made of a
material selected from the group consisting of Ag, Ni, Al, Ph, Pd,
Ir, Ru, Mg, Zn, Pt, Au and combinations thereof, and preferably has
a reflectivity of 70% or more in order to lower the contact
resistance as well as satisfy high reflectivity conditions.
[0040] Further, the high reflectivity Ohmic contact layer may be
preferably constituted of a first layer made of a material selected
from the group consisting of Ni, Pd, Ir, Pt and Zn and a second
layer of Ag or Al formed on the first layer. Alternatively, the
high reflectivity Ohmic contact layer may have a first layer made
of Ni, a second layer of Ag formed on the first layer and a third
layer of Pt formed on the second layer.
[0041] Herein, it is preferred that the first layer has a thickness
ranging from about 5 to 50 .ANG., the second layer has thickness
ranging from about 1000 to 10000 .ANG., and the third layer has a
thickness ranging from about 100 to 500 .ANG. in order to improve
the high reflectivity of the Ohmic contact layer having a two or
three layered structure.
[0042] The high reflectivity Ohmic contact layer is formed via
conventional vapor deposition or sputtering, in particular, at a
temperature ranging from about 400 to 900.degree. C. in order to
improve the properties of the Ohmic contact layer.
[0043] P-Bonding Electrode and N-Electrode
[0044] The bonding electrode constitutes a component of the
p-electrode structure together with the high reflectivity Ohmic
contact layer and the metal barrier layer. The bonding electrode
functions as the outermost electrode layer to be mounted on a lead
pattern via conductive bumps in a flip chip structure, and is made
of Au or an alloy containing Au.
[0045] Further, the n-electrode formed on the n-doped nitride
semiconductor layer may be formed of a single or multiple layer
structure that is made of a material selected from the group
consisting of Ti, Cr, Al, Cu and Au.
[0046] Such electrodes may be formed via a typical method for
growing metal layers such as vapor deposition and sputtering.
[0047] Metal Barrier Layer
[0048] The metal barrier layer of the invention, which is formed on
a high reflectivity Ohmic contact layer area in which the p-bonding
electrode is to be formed, serves as a layer for mixing in the
interface between a bonding electrode material and an Ohmic contact
material to prevent any property deterioration of the Ohmic contact
layer (such as reflectivity and contact resistance). The metal
barrier layer of this type may be constituted of a single or
multiple layer structure that is made of a material selected from
the group consisting of Ni, Al, Cu, Cr, Ti and combinations
thereof.
[0049] Alternatively, the metal barrier layer may be extended to
the sides of the high reflectivity Ohmic contact layer according to
an alternative embodiment. In particular, the alternative
embodiment has an advantage in that the metal barrier layer can
effectively prevent the current leakage that is originated from the
migration of Ag if any contained in the high reflectivity Ohmic
contact layer.
[0050] In addition, the metal barrier layer of a predetermined
reflectivity may be formed also in contact with the p-doped nitride
semiconductor layer in the open areas of the high reflectivity
Ohmic contact layer of a mesh structure in order to assist the
reflection of the high reflectivity Ohmic contact layer.
[0051] The metal barrier layer is formed via conventional vapor
deposition or sputtering as other electrodes, and preferably, heat
treated at a temperature of about 300.degree. C. for tens of
seconds to several minutes in order to improve bonding ability.
[0052] Dielectric Barrier Layer
[0053] The dielectric barrier layer adoptable in the invention may
be selectively formed on the high reflectivity Ohmic contact layer
to expose at least the p-bonding electrode.
[0054] Preferably, the dielectric barrier layer may be formed to
the extent of surrounding the sides of the high reflectivity Ohmic
contact layer in order to serve as a typical passivation layer
while preventing the current leakage that is originated from the
migration of Al in the high Ohmic contact layer. According to an
alternative embodiment, the dielectric barrier layer may be formed
on one side of the LED, likewise to a conventional passivation
structure, in order to expose the p-bonding electrode and the
n-electrode.
[0055] The dielectric barrier layer may be formed of oxide or
nitride containing an element selected from the group consisting of
Si, Zr, Ta, Ti, In, Sn, Mg and Al.
[0056] Preferably, the dielectric barrier layer may be constituted
of a high reflectivity layer having two types of dielectric layers
of different refractivity, alternating with each other. The high
reflectivity dielectric barrier layer in the flip chip LED may be
formed according to a fabrication method for dielectric mirror
layers disclosed in Korean Patent Application Serial No. 2003-41172
by Samsung Electro-Mechanics Co. (Jun. 24, 2003).
[0057] Hereinafter embodiments of the invention will be described
in more detail with reference to the accompanying drawings.
[0058] FIG. 2A is a side sectional view illustrating a flip chip
nitride semiconductor LED 30 according to a first embodiment of the
invention, and FIG. 2B is a plan view of FIG. 2A.
[0059] Referring to FIG. 2A first, the flip chip nitride
semiconductor LED 30 includes a substrate 31 of for example
sapphire for growing nitride semiconductors thereon and an n-doped
nitride semiconductor layer 32, an active layer 33 and a p-doped
nitride semiconductor layer 34 formed in their order on the
substrate 31.
[0060] The nitride semiconductor LED 30 has an n-electrode 39a
provided on a top region of the n-doped nitride semiconductor layer
32 which is exposed via mesa etching. A p-electrode structure of
the nitride semiconductor LED 30 includes a high reflectivity Ohmic
contact layer 35, a metal barrier layer 36 and a bonding layer 39b.
The high reflectivity Ohmic contact layer 34 is formed on the
p-doped nitride semiconductor layer 34, and has a mesh structure
having a number of open areas to partially expose the p-doped
nitride semiconductor layer 34. Further, the metal barrier layer 36
is formed on a predetermined top region of the high reflectivity
Ohmic contact layer 35 on which the bonding electrode 39b is to be
formed.
[0061] The high reflectivity Ohmic contact layer 34 preferably has
a reflectivity of 70% or more, and forms an Ohmic contact with the
p-doped nitride semiconductor layer. The high reflectivity Ohmic
contact layer may have at least one layer made of a material
selected from the group consisting of Ag, Ni, Al, Ph, Pd, Ir, Ru,
Mg, Zn, Pt, Au and combination thereof. Preferably, the high
reflectivity Ohmic contact layer 34 may be made of one of Ni/Ag,
Zn/Ag, Ni/Al, Zn/Al, Pd/Ag, Pd/Al, Ir/Ag, Ir/Au, Pt/Ag, Pt/Al and
Ni/Ag/Pt.
[0062] Referring to FIG. 2B together with FIG. 2A, a person of
ordinary skill may understand the principle that current crowing is
alleviated by the high reflectivity Ohmic contact layer 35 of a
mesh structure according to the invention. That is, owing to the
characteristics of the mesh structure as shown in FIG. 2B, current
flows through the high reflectivity Ohmic contact layer 35, which
has a specific resistance lower than that of the p-doped nitride
semiconductor layer 34, along a long path (for example indicated
with an arrow in FIG. 2B) until it reaches the n-electrode 39a.
Therefore, the ratio of current directly flowing toward the p-doped
nitride semiconductor layer 34 as shown in FIG. 2A can be
relatively increased. As a result, this alleviates current crowding
and at the same time allows more uniform light to be emitted from
the entire active layer 33 thereby to remarkably increase the
luminous efficiency while improving the reliability of the LED.
[0063] Although the high reflectivity Ohmic contact layer having
the mesh structure of the invention can sufficiently alleviate
current crowding as it is, the total open areas can correspond to
preferably 70% or less, and more preferably, 50% or less of the
entire top area of the Ohmic contact layer 35 in order to obtain a
sufficient reflection area and improve current injection
efficiency.
[0064] FIG. 3A is a side sectional view illustrating a flip chip
nitride semiconductor LED 50 according to a second embodiment of
the invention.
[0065] As shown in FIG. 3A, the flip chip nitride semiconductor LED
50 includes a substrate 51 of for example sapphire for growing
nitride semiconductors thereon and an n-doped nitride semiconductor
layer 52, an active layer 53 and a p-doped nitride semiconductor
layer 54 formed in their order on the substrate 51.
[0066] The nitride semiconductor LED 50 also has an n-electrode 59a
and p-electrode structure. The n-electrode 59a is provided on a top
region of the n-doped nitride semiconductor 52 exposed via mesa
etching. The p-electrode structure includes a high reflectivity
Ohmic contact layer 55, a metal barrier layer 56 and a bonding
electrode 59b similar to the p-electrode structure shown in FIG.
2A, in which the metal barrier layer 56 is so configured to
surround or cover not only the high reflectivity Ohmic contact
layer 55 but also the sides thereof. While the metal barrier layer
36 in FIG. 2A is merely expectable to function as a barrier for
preventing Au components from mixing in the interface between the
p-bonding electrode 39b and the high reflectivity Ohmic contact
layer 35, the metal barrier layer 56 of this embodiment can be
expected to prevent the current leakage that is originated from the
migration of elements in the high reflectivity Ohmic contact layer
55. In particular, this embodiment can be advantageously applied in
case that the high reflectivity Ohmic contact layer 55 contains a
high mobility element such as Ag.
[0067] FIG. 3B is a side sectional view illustrating a chip
structure or flip chip light emitting device 60 having the flip
chip nitride semiconductor LED 50 in FIG. 3A mounted thereon.
[0068] As shown in FIG. 3B, the nitride semiconductor LED 50 can be
mounted on a supporting substrate 61 by welding electrodes 69a and
69b with lead patterns 62a and 62b via conductive bumps 64a and
64b. As described hereinbefore, the sapphire substrate 51 of the
LED 50 in the flip chip light emitting device 60 is used as a light
emitting plane since it is transparent. The high reflectivity Ohmic
contact layer 55 can lower the contact resistance between itself
and the p-doped nitride semiconductor layer 54, and owing to the
high reflectivity, increase light quantity directed toward the
light emitting plane. Further, since the metal barrier layer 56 of
the LED 50 having a specific reflectivity is so formed to cover
open areas of the high reflectivity Ohmic contact layer 55, it can
improve the total reflection ability thereby to realize higher
luminance.
[0069] FIG. 4A is a side sectional view illustrating a flip chip
nitride semiconductor LED 70 according to a third embodiment of the
invention.
[0070] As shown in FIG. 4A, the flip chip nitride semiconductor LED
70 includes a sapphire substrate 71 and an n-doped nitride
semiconductor layer 72, an active layer 73 and a p-doped nitride
semiconductor layer 74 formed in their order on the sapphire
substrate 71. The nitride semiconductor LED 70 also includes an
n-electrode 79a formed on a top region of the n-doped nitride
semiconductor layer 72, which is exposed via mesa etching, and a
p-electrode structure having a high reflectivity Ohmic contact
layer 74, a metal barrier layer 75 and a bonding electrode 79b
likewise to the electrode structures shown in FIG. 2A.
[0071] In this embodiment, a metal barrier layer 76 is provided to
function as a barrier for preventing Au components from mixing in
the interface between the p-bonding electrode 39b and the high
reflectivity Ohmic contact layer 35. A dielectric barrier layer 77
is also provided to prevent the current leakage originated from the
migration of high mobility elements such as Ag in the high
reflectivity Ohmic contact layer 75. Alternatively, the dielectric
barrier layer 77 may be extended to the sides of the LED 70 to
expose the p-bonding electrode 79b and the n-electrode 79a as a
conventional passivation layer. The dielectric barrier layer 77 may
be made of oxide or nitride containing an element selected from the
group consisting of Si, Zr, Ta, Ti, In, Sn, Mg and Al.
[0072] The dielectric barrier layer 77 of the invention may be
formed into a dielectric mirror structure of high reflectivity to
remarkably improve the entire reflection ability of the LED. That
is, the dielectric barrier layer 77 of the invention is constituted
of two types of dielectric layers of different refractivity,
repeatedly alternating with each other.
[0073] FIG. 4B is an enlargement of a part B of the dielectric
barrier layer 77 in FIG. 4A. As shown in FIG. 4B, the dielectric
barrier layer can have a structure obtained by repeatedly
alternating SiO.sub.2 films and Si.sub.3N.sub.4 films of different
refractivity with each other. Such a method for forming the high
reflectivity dielectric barrier layer into a dielectric mirror
structure may be adopted from a fabrication method for dielectric
mirror layers disclosed in Korean Patent Application Serial No.
2003-41172 by Samsung Electro-Mechanics Co. (Jun. 24, 2003).
According to this document, a high reflectivity layer can be
obtained with a reflectivity of 90% or more, and more preferably,
97% or more. With the dielectric barrier layer obtained by
repeatedly alternating the dielectric layers of different
refractivity with each other as described hereinbefore, the
invention can remarkably improve the reflection ability of the
entire LED and thus increase the effective luminous efficiency of
the flip chip nitride semiconductor LED up to a very high
level.
EXAMPLES
[0074] Experiments were carried out as follows in order to compare
properties of flip chip nitride semiconductor LEDs according to
Inventive Examples with those of a conventional flip chip nitride
semiconductor LED according to a Comparative Example.
Inventive Example 1
[0075] First, after loading a sapphire substrate into a MOCVD
chamber, a GaN low temperature nucleation layer was grown as a
buffer layer. Then, an n-doped semiconductor layer of an n-doped
GaN film and an n-doped AlGaN layer, an active layer having a
multiple quantum well structure of InGaN/GaN films and a p-doped
nitride semiconductor layer of a p-doped GaN film were formed on
the buffer layer to obtain a blue LED.
[0076] Next, a high reflectivity Ohmic contact layer of a mesh
structure having an open area ratio of about 30% was formed on the
p-doped nitride semiconductor layer, and then a resultant structure
was heat treated at a temperature of about 500.degree. C. The high
reflectivity Ohmic contact layer of the Inventive Example 1 was
made from Ni/Ag. Herein the open area ratio means the ratio of open
areas with respect to the entire area (i.e., the area surround by
the outermost periphery) as generally used in the
specification.
[0077] Then, a metal barrier layer on the p-doped nitride
semiconductor layer was formed to surround the top and sides of the
high reflectivity Ohmic contact layers (refer to FIG. 3A), and a
resultant structure was heat treated at a temperature of about
350.degree. C. Next, a p-bonding electrode and an n-electrode
containing Au in common were formed to complete a flip chip nitride
semiconductor LED.
[0078] The flip chip nitride semiconductor LED prepared in the
Inventive Example 1 was coupled with a supporting substrate
provided with a lead pattern as shown in FIG. 3B to produce a flip
chip light emitting device.
Inventive Example 2
[0079] In the Inventive Example 2, a flip chip nitride
semiconductor LED was prepared according to the same conditions as
in the Inventive Example 1 except that a high reflectivity Ohmic
contact layer of a mesh structure was patterned to have an open
area ratio of about 50%. The flip chip nitride semiconductor LED
prepared like this was coupled with a supporting substrate provided
with a lead pattern as shown in FIG. 3B to produce a flip chip
light emitting device.
Inventive Example 3
[0080] In the Inventive Example 3, a flip chip nitride
semiconductor LED was prepared according to the same conditions as
in the Inventive Example 1 except that a high reflectivity Ohmic
contact layer of a mesh structure was patterned to have an open
area ratio of about 70%. The flip chip nitride semiconductor LED
prepared like this was coupled with a supporting substrate provided
with a lead pattern as shown in FIG. 3B to produce a flip chip
light emitting device.
Comparative Example
[0081] In this Comparative Example, a flip chip nitride
semiconductor LED was prepared according to the same conditions as
the above Inventive Examples 1 to 3 except that a high reflectivity
Ohmic contact layer was formed into a conventional configuration
(or unmeshed configuration) of the same entire area without any
separate patterning process for forming a mesh structure. The flip
chip nitride semiconductor LED prepared like this was coupled with
a supporting substrate provided with a lead pattern as shown in
FIG. 3B to produce a flip chip light emitting device.
[0082] The flip chip light emitting devices produced according to
the Inventive Examples 1 to 3 and the Comparative Example as above
were measured of forward voltage characteristics and electric power
characteristics. FIGS. 5A and 5B are graphs comparing the forward
and output voltage characteristics of the flip chip light emitting
devices according to the Inventive Examples 1 to 3 and the
Comparative Example.
[0083] Referring to FIG. 5A, the flip chip light emitting device of
the Comparative Example showed a forward voltage of about 3.42V,
whereas the flip chip light emitting devices according to the
Inventive Examples 1 to 3 showed forward voltages of about 3.10V,
3.11V and 3.12V, respectively. This indicates that the Inventive
Examples 1 to 3 had average voltage reduction of about 0.3V and
thus forward voltage characteristics improvement of about 10%.
Further, the Inventive Example 1 (in which the open area ratio was
30%) showed the lowest forward voltage of 3.10V.
[0084] Referring to FIG. 5B, the flip chip light emitting device of
the Comparative Example showed an electric power of about 18.53 mW,
whereas the flip chip light emitting devices according to the
Inventive Examples 1 to 3 showed electric powers of about 20.59 mW,
19.99 mW and 19.24 mW, respectively. In particular, the Inventive
Example 1 (in which the open area ratio was 30%) obtained power
enhancement of about 2 mW and thus efficiency improvement of 10% or
more over the Comparative Example.
[0085] While the present invention has been described with
reference to the particular illustrative embodiments and the
accompanying drawings, it is not to be limited thereto but will be
defined by the appended claims. It is to be appreciated that those
skilled in the art can substitute, change or modify the embodiments
into various forms without departing from the scope and spirit of
the present invention.
[0086] As described hereinbefore, the nitride semiconductor LED of
the present invention adopts the high reflectivity Ohmic contact
layer of a mesh structure in the p-electrode structure to decrease
current concentrated on a region of the LED adjacent to the
n-electrode as well as increase current flowing toward the p-doped
nitride semiconductor layer thereby reducing current crowding. As a
consequence, the flip chip nitride semiconductor LED of the
invention can have lower forward voltage and higher luminous
efficiency while effectively preventing deterioration thereby to
remarkably improve reliability.
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